Back then, I haven't encountered any issues with him and I didn't want to be harsh to him replying something like "I don't want to answer your PM spam" / "what are you bothering me with it, just ask it in Reputation, no need to spam my inbox".
Or leaving his PM's completely unanswered.

In general, I'm always trying to be nice to everyone, who's sending me PM's as long as it's not an outright troll.
I've even answered numerous shitposters, who sent me PM's and asked me to remove my negative trust that they've received due to signature campaign related abuses. And I'm trying to stay friends for most of the time.
So, I just want to be a nice person and address all legitimate PM's even sometimes these PM's are just unnecessary. Being friendly is always recommended.

Well, now I know better. I won't reply to any PM from him next time and report it as unsolicited PM spam right away.  

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Maybe he starts realizing that his current approach to DT isn't appreciated by the community. Most likely, it's my personal wishful thinking but even trolls can improve.  
I'm sure that if he's apologizing for what he did, maybe some DT members will remove him from his distrust list.  
But that's not up to me to decide. After what he has done recenly, he truly deserves to get a ton of more distrusts and if he continues, he might get even more.

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+1
Important point.